The proposed research seeks to define the structural parameters which determine the redox potentials of hemeproteins. Comparative biochemical research has revealed that hemeproteins exhibit an extremely wide range of redox potentials. Previous studies of model- heme complexes have indicated that differences between heme moieties and extra planar ligands to the heme iron represent significant parameters for determining the redox potentials of hemeproteins. Yet hemeproteins that have the same heme moiety and protein ligands exhibit a wide range of redox potentials. Furthermore even larger differences may be observed between the potentials of these hemeproteins and model heme ligand complexes. Thus a fundamental question has been raised regarding the structural basis for the redox potentials of hemeproteins. Toward a solution of this problem this investigator has recently considered the effect of the heme environment on its redox potential. Redox potentials of model heme complexes in a non-polar solvent together with theoretical calculations have led to the proposal that differences between the redox potentials of hemeproteins and model heme complexes could be accounted for by the relative hydrophobicities of the heme environments. Solvent perturbation studies of hemeproteins whose redox potentials span a range of values are proposed to measure the extent of exposure of the heme group to the solvent. The extent of exposure of a heme group to solvent is related to the hydrophobicity of its environment. The experiments seek to provide a quantitative measure of the relative hydrophobicities of the heme environments in order to test the validity of the proposed theory.